I wish I could have been satisfied with 3-4 engines and 1-2 dozen pieces of rolling stock, but my (our) brain isn't wired that way...gotta have more.
I started in HO, had a 12x16 layout until the varmints invaded my shed plus walking to/from it (behind my garage and dark at night) drove me away. After about 5 years I developed an interest in O, so I bought 1 engine (MTH RS3). That sat for a year before I started tinkering with the idea of a layout in my garage, ended up with a 7x16 layout.
Having to go out to either turn the heat or the AC on before I ran trains prompted me to move indoors, 2nd floor of the small house I have. That gave me a 13x30 layout, but it was 3-rail.
A few years ago I went from 3-rail DCS/TMCC to 3-rail BPRC, then finally when I tore everything down and rebuilt with Mianne benchwork I went with 3-rail engines and rolling stock on 2-rail code 148 track...never looked back.
I have way too many engines for my taste, if I had seen into the future I would have waited until the price of Williams brass came down and bought only steam engines. 3-4 engines would be about all I truly need: switcher, 2 mainline engines, and a passenger engine.
What I'm getting at is this...If a person (me) could just control themselves, O-scale could be very affordable.
